Buying tickets

Yeah, I've never been to any international before, but today we're looking in to getting some tickets, and I was just wondering if any of you who HAVE been before know any good places to get them from? Like, any reputable ticket selling sites or something like that, or anywhere really. We're going to go to the west indies for a holiday next year and my family says if there are tickets available we may as well go catch a match while we're there, it'd be good. But if we can't, we're still looking to go see a test match next summer here in England, so yeah. I'd much appreciate any advice anyone could offer.